The invention relates to a method for continuously cleaning a heat exchanger during operation as well as to an apparatus to be used with such a method.
More specifically, the invention relates to a method for continuously cleaning a heat exchanger of a type called the closed loop type, which is provided with a series of heat exchanging pipes, with one medium--for instance the cooling medium--passing through the pipes and the other medium--for instance the medium to be cooled--being carried along the pipes. Heat exchangers of this type are used on a large scale in many branches of industry, for instance in the petroleum and coal industries for cooling the products obtained from hydrocrackers and gasifiers. A cooling medium often used is water or air. When air is used, the cooling medium is usually passed through the heat exchanging pipes while the air is blown along the pipes at a high velocity. In a heat exchanger in which water is used as the cooling medium the water is usually carried through the pipes while the medium to be cooled flows along the pipes.
The invention relates to a method and apparatus for continuously cleaning a heat exchanger used for cooling a gaseous medium which is polluted by solid particles. Such a gaseous medium to be cooled may be for instance product gas obtained from the partial combustion of liquid or solid hydrocarbons. Such product gases usually contain fairly large quantities of small to very small solid particles, such as soot and fly ash. Particularly when the solid particles are somewhat sticky there is a risk of these particles adhering to the walls of the heat exchanging pipes when, along with the gas to be cooled, they are carried through a heat exchanger. However, such a particle buildup on the pipe walls will soon lead to a decrease in the rate of heat transfer between gas to be cooled and cooling medium. When the heat transfer efficiency of the heat exchanger has fallen to a certain level, the heat exchanging pipes have to be cleaned in order to restore their efficiency.
In practice, a vast variety of methods and devices are used for cleaning the surfaces of heat exchanging pipes. A well known cleaning method comprises passing solid particles, for instance grains of sand and tiny steel balls, along or through the heat exchanging pipes. During their passage these solid particles strike against the pipe walls and thus remove deposits from the pipe walls. The solid cleaning particles can be introduced into the heat exchanger during operation, which obviates the need for shutting down the heat exchanger for a turn-out.
If in case of severely polluted gases a heat exchanger is to maintain a constant maximum heat transfer efficiency, the pipe walls must preferably be cleaned continuously. According to the known method the continuous cleaning of the pipe walls can be performed by moving a stream of solid particles together with the gases in continuous circulation through the heat exchanger. In case of a heat exchanger used for cooling gas which is polluted by solid particles, the solid cleaning particles are preferably passed through the heat exchanger together with the gas stream forcing the solid cleaning particles along. When the gas containing the cleaning particles has left the heat exchanger, it is passed through a separator in order to remove the cleaning particles together with the entrained solid impurities from the gas stream. The separated cleaning particles may subsequently be recirculated to the heat exchanger to perform another cleaning cycle. In the abovementioned known method of continuously cleaning heat exchangers the solid particles are circulated by means of mechanical pumping. Particularly the use of rigid cleaning particles, such as sand grains, leads to a great deal of wear in the circulating pump due to the scouring effect of the solid particles.
According to another known method for continuously cleaning vertical pipe walls of a heat exchanger, solid cleaning particles are provided inside or outside the heat exchanging pipes in such a manner that, during operation, a fluidized bed is created by an upward flow of the heat absorbing or the heat emitting medium. This method has the advantage over the aforementioned method that the particles remain in the heat exchanger permanently and that therefore the medium carried along those particles need not be subjected to further treatment for separating the medium from the cleaning particles. However, the latter method does have a number of disadvantages, for instance the possibility of the fluidized bed of cleaning particles becoming choked by impurities, instability of the bed in case of fluctuations of the medium passing through the bed during operation, as well as the limited possibility of working at reduced throughput rates, since a certain minimum velocity of the medium is required to prevent the fluidized bed from collapsing.
It is an object of the invention to provide an improved method of continuously cleaning a heat exchanger, which does not require the use of mechanical pumping devices that can easily be damaged, and by which the solid particles themselves are continuously cleaned, so that the cleaning particles in the heat exchanger will produce an optimum effect which will also be maintained with none of the drawbacks adhering to the last-named clearing method.
It is another object of the invention to provide an apparatus to be used with such an improved cleaning method.